Load cups are used to ensure that a substrate is properly aligned and positioned so that a polishing head of a chemical-mechanical planarization (CMP) system can pick up the substrate properly. An example of such a CMP system including a load cup is described in detail in U.S. Pat. No. 7,044,832, issued May 16, 2006 to Yilmaz et al. which is hereby incorporated by reference for all purposes. Sensors are typically used to determine if a substrate is present within a load cup. However, due to a number of factors, the relatively low cost sensors that are typically used, may fail prematurely. Thus, what is needed are improved methods and apparatus for reliably sensing the presence of a substrate in a load cup that does not significantly increase the cost of manufacturing the CMP system.